Because of its clean burning qualities and convenience, natural gas has become widely used in recent years. Many sources of natural gas are located in remote areas, great distances from any commercial markets for the gas. Sometimes a pipeline is available for transporting produced natural gas to a commercial market. When pipeline transportation is not feasible, produced natural gas is often processed into liquefied natural gas (which is called “LNG”) for transport to market.
In designing an effective and efficient LNG plant, that is an industrial process facility designed to conduct the conversion of natural gas, from gaseous form to liquid, many refrigeration cycles have been used to liquefy natural gas by cooling. The three types most commonly used in LNG plants today are: (1) the “cascade cycle,” which uses multiple single component refrigerants in heat exchangers arranged progressively to reduce the temperature of the gas to a liquefaction temperature; (2) the “multi-component refrigeration cycle,” which uses a multi-component refrigerant in specially designed exchangers; and (3) the “expander cycle,” which expands gas from feed gas pressure to a low pressure with a corresponding reduction in temperature. Variants of the last cycle, the expander cycle, have been found to provide substantial contribution to the state of the art, see WO-A-2007/021351, published 22 Feb., 2007. As described here, using a portion of the feed gas stream in a high pressure expander loop can contribute a refrigerant stream for heat exchange treatment of that feed gas and this largely permits the elimination of external refrigerants while improving overall efficiencies.
However, though a significant improvement over prior art processes using expander cooling cycles, the process of WO-A-2007/021351 can still suffer thermodynamic inefficiencies, particularly where high local ambient temperatures prevent effective use of ambient temperature air or water cooling to achieve effective lowering of the temperatures of process gas or liquid streams. And, where colder water is theoretically available in lower depths of water even though ambient surface temperatures are high, there may be significant costs associated with placing and operating access piping for carrying deep waters to a LNG platform, specifically floating production system. The constant movement of a floating production system places stresses and strains on pivoted piping extending down from the platform, thus raising structural support problems. Also the amount of water needed can require high horsepower pumps if the depth is much below the surface, obviously increasing with the depth of the cooler water sought.
The goal for LNG liquefaction process development is to try to match the natural gas cooling curve with the refrigerant warming curve. For liquefaction systems based on refrigerants, this means splitting the refrigerant into two streams which are cooled to different temperatures. Typically, the cold end is cooled by a refrigerant whose composition is chosen such that the warming curve best matches the natural gas cooling curve for the cold temperature range. The warm end is typically cooled with propane for economic reasons but again a refrigerant with a chosen composition may be used to better match the natural gas cooling curve for the warm end. Furthermore, for liquefaction processes operating at high ambient temperatures, the pre-cooling (warm end) refrigeration system would become excessively large and costly. In the process of WO-A-2007/021351, this may represent over 70% of the installed compression horsepower. The classic approach is to further split the cooling temperature range and add another refrigeration loop. This is typical of the cascade liquefaction cycle which typically involves three refrigerants. This adds to the complexity of the process and results in increased equipment count as well as cost.
Accordingly, there is still a need for a high-pressure expander cycle process providing improved efficiencies where ambient temperatures of air and water do not provide sufficient cooling to minimize power required and the costs therewith for the overall cycle. In particular a process that can reduce the overall horsepower requirements of natural gas liquefaction facility, particularly one operating in high ambient temperatures is still of high interest.
Other related information may be found in International Publication No. WO2007/021351; Foglietta, J. H., et al., “Consider Dual Independent Expander Refrigeration for LNG Production New Methodology May Enable Reducing Cost to Produce Stranded Gas,” Hydrocarbon Processing, Gulf Publishing Co., vol. 83, no. 1, pp. 39-44 (January 2004); U.S. App. No. US2003/089125; U.S. Pat. No. 6,412,302; U.S. Pat. No. 3,162,519; U.S. Pat. No. 3,323,315; and German Pat. No. DE19517116.